1. Field of the Invention
This invention relates to a process for producing toner particles having superior electrophotographic performance in each particle of which a colorant and a release agent or a mixture of these stand(s) dispersed finely and uniformly. This invention also relates to a process by which the toner particles having superior electrophotographic performance in each particle of which a colorant and a release agent or a mixture of these stand(s) dispersed finely and uniformly are produced controlling any excess power consumption.
This invention still also relates to a process by which toner particles containing no coarse particles and having sharp particle size distribution are obtained in a high efficiency.
2. Related Background Art
Conventionally, electrophotography is a process in which fixed images are obtained by forming an electrostatic latent image on a photosensitive member by various means, subsequently developing the latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by the action of heat, pressure, heat and pressure or solvent vapor to form a fixed image.
Toners used therefor are commonly produced by melt-kneading a colorant into a thermoplastic resin to effect dispersion uniformly, thereafter cooling the resultant melt-kneaded product to solidify, finely pulverizing the kneaded product by means of a fine grinding mill, and classifying the finely pulverized product by means of a classifier to obtain toner particles having the desired particle diameters, followed by external addition of stated additives to produce a toner.
Reasonably good toners can be produced by such a production method, but there is a limit to the range in which toner materials are selected. For example, in the case when the toner particles are produced by pulverization, kneaded products must be brittle enough to be pulverizable by means of economically available production apparatus. Kneaded products made brittle in order to meet these requirement tend to result in a broad particle size range of the particles formed when actually pulverized at a high speed, especially causing a problem that fine particles tend to be included in the particles in a relatively large proportion. Moreover, toners making use of such highly brittle materials tend to be further pulverized or powdered when used in, e.g., copying machines.
In this pulverization method, it is also not easy to uniformly disperse solid fine particles of colorants or the like in the resin, and, depending on the degree of their dispersion, toners may cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency when images are formed. Accordingly, care must well be taken when colorants are dispersed. Also, colorants may come bare to surfaces of toner particles to cause fluctuations in developing performance of toners.
Meanwhile, in order to overcome the problems of the toners produced by such pulverization, toners produced by suspension polymerization and other various polymerization toners, as well as methods of producing such toners are proposed as disclosed in Japanese Patent Application Laid-open No. 51-14895 and so forth. For example, in the suspension polymerization, a colorant is dispersed in a polymerizable monomer to obtain a polymerizable monomer mixture, and thereafter a polymerization initiator and also optionally a cross-linking agent, a charge control agent and other additives are uniformly dissolved or dispersed to form a polymerizable monomer composition. Thereafter, this polymerizable monomer composition is dispersed in an aqueous medium containing a dispersion stabilizer, by means of a suitable agitator, and is simultaneously subjected to polymerization to polymerize the polymerizable monomer to obtain toner particles having the desired particle diameters.
Since this method has no step of pulverization, the toner particles are not required to be brittle, and hence soft resins can be used as the resin constituting the toner particles. Also, colorants can not easily come bare to the surfaces of toner particles and hence the toner particles can have a uniform triboelectric charging performance. This method has such advantages. Also, since the toner particles obtained have a relatively sharp particle size distribution, the step of classification can be omitted, or even when classification is carried out, the toner particles can be obtained in a high yield.
This method also has another advantage that a low-softening materials as a release agent can be encapsulated in toner particles in a large quantity and in plural kinds and hence the toner particles obtained can have excellent anti-offset properties.
In the production of toner particles by polymerization, it is important to make the particulate, colorant or release agent or the both sufficiently dispersed or dissolved in the liquid, polymerizable monomer. Accordingly, it is common to make the polymerizable monomer serve as a liquid medium and employ a dispersion step of dispersing therein the colorant or release agent or the both. Various apparatus are also known as dispersion machines used in such a dispersion step.
In the past, as disclosed in Japanese Patent Application Laid-open No. H10-232510, a method has been proposed in which an agitation media type dispersion machine is used in order to disperse the colorant in the polymerizable monomer in the form of fine particles to obtain a fluid mixture. As also disclosed in Japanese Patent No. 3298443 and Japanese Patent Applications Laid-open No. H6-273977 and No. H8-134359, a method is proposed in which the agitation media type dispersion machine is used in order to disperse in the polymerizable monomer a release agent which is substantially insoluble therein at normal temperature or within the range of temperature having been controlled during production, to obtain a fluid mixture.
However, in such an agitation media type dispersion machine, the particles of colorant or the particles of release agent are dispersed or crushed by the aid of their collision against media and shear stress between them, and hence kinetic energy must be applied to the media by agitation. This energy is so large as to cause a rise in production cost for toner particles. Also, the energy applied is not only used in dispersion or crushing but also used as the generation of heat that is caused by collision between media and by collision of media against a machine casing. This generation of heat may adversely affect the polymerizable monomer, e.g., make it undergo thermal polymerization. Accordingly, the machine casing is set up in a jacket structure to keep the heat from being generated, or a heat exchanger is provided outside the machine to eliminate the heat generated. Excess heat generation caused by agitation results in wasteful use of energy.
Especially in recent years, media with small diameter (stated specifically, media of 0.05 mm to 2 mm in diameter) are often used in order to improve the degree of dispersion in the agitation media type dispersion machine. The use of such media with small diameter enables the media to be densely packed, and hence the degree of dispersion can dramatically been improved. This, however, has caused a problem that the agitation power increases correspondingly. In addition, friction tends to be produced between the media and the machine casing surface with which the former comes into contact, bringing about a possibility that wear dust contaminates toner particles more frequently.
Without using such an agitation media type dispersion machine consuming a large power (electric power), a medialess dispersion machine (a dispersion machine commonly called a colloid mill) may also be used to restrain excess power consumption, but it has not been easy to achieve the desired dispersion.
Further, in the production of toner particles by polymerization, it is important, in view of characteristic features of the production process, that fine particles of a polymerizable monomer composition which have the desired particle diameter and a sharp particle size distribution are obtained in the step of granulation. In the past, in this step of granulation, continuous granulation has been proposed, aiming at simplification of the production steps. This continuous granulation is a method in which the polymerizable monomer composition is continuously fed to a granulating machine to obtain a cluster of droplets of the polymerizable monomer composition in the desired size, and thereafter an aqueous medium containing the cluster of droplets obtained is taken out, which is then led to a polymerization bath, where the polymerization is completed to obtain polymer particles. Such a method is disclosed in Japanese Patent No. 3248747 and Japanese Patent Application Laid-open No. 2001-255697.
The granulating machine used in this method is an apparatus called a colloid mill, constituted of agitating blades rotated by means of a rotating shaft, and a discharge-controlling gap provided around them. In this discharge-controlling gap, a polymerizable monomer composition ejected by an ejection force produced by the rotation of the blades is agitated by a shear force to come to be the desired fine particles. The granulating machine having such a discharge-controlling gap has commonly been used in the continuous granulation. As this gap, a gap of about 1 to 10 mm is provided in order to avoid contact with the agitating blades rotated at a high speed. Taking account of shaft run-out of the rotating shaft and its mechanical precision, it is difficult to narrow the gap more than this. However, toners for electrophotography have particle diameters of 3 to 15 μm, and the shear force produced at the gap that is hundreds of times to thousands of times the particle diameters is not enough to effect sufficient continuous granulation, to cause short pass or shortage of shear force, making it difficult to achieve the desired toner particle diameter and particle size distribution.